Ceph Storage Calculator

Estimate your cluster’s usable capacity based on your hardware and data protection configuration.

What is a Ceph Storage Calculator?

A ceph storage calculator is a specialized tool designed for system administrators, storage architects, and IT professionals to plan and estimate the capacity of a Ceph storage cluster. Ceph is a powerful, open-source, software-defined storage platform that is highly scalable and has no single point of failure. However, understanding the relationship between the raw physical disk space and the actual usable space can be complex. This calculator simplifies the process by factoring in key variables like the number of disks, individual disk size, and the chosen data protection method (replication).

Planning is crucial because the raw capacity you purchase is not the amount of data you can store. Data redundancy, which is essential for fault tolerance, consumes a significant portion of the total space. This tool helps you make informed hardware purchasing decisions and set realistic expectations for your Ceph capacity planning before deployment.

Ceph Capacity Formula and Explanation

The fundamental calculation for determining usable space in a replicated Ceph pool is straightforward. The calculator uses this formula to provide its estimates:

Usable Capacity = Raw Capacity / Replication Factor

Where:

• Raw Capacity is the total physical storage across all disks.
• Replication Factor is the number of copies of data you wish to maintain for redundancy. A factor of 3 is standard for production environments, meaning for every piece of data written, two additional copies are created.

This calculator also shows “Effective Usable Capacity,” which assumes a standard 85-90% utilization to leave headroom for cluster maintenance, rebalancing, and to prevent performance degradation when the cluster gets too full. It is a critical mistake to plan on using 100% of your usable space.

Key Variables for Ceph Capacity Calculation

Variable	Meaning	Unit	Typical Range
Number of OSDs	The total count of individual disks dedicated to Ceph storage.	Unitless	3 – 1000+
Disk Size	The storage capacity of a single OSD.	TB or GB	1 TB – 22 TB
Replication Factor	Total copies of each data object.	Integer	2 – 4 (3 is most common)
Usable Capacity	The space available for data storage after accounting for replication.	TB or GB	Depends on inputs

Practical Examples

Example 1: Small Homelab Cluster

A user is setting up a small, budget-friendly cluster for a homelab or testing environment.

• Inputs:
  • Number of OSDs: 4
  • Size per Disk: 2 TB
  • Replication Factor: 2 (less redundancy for non-critical data)
• Results:
  • Raw Capacity: 8 TB (4 disks * 2 TB)
  • Usable Capacity (Pre-Overhead): 4 TB (8 TB / 2)
  • Effective Usable Capacity (~85%): ~3.4 TB

Example 2: Production Enterprise Cluster

An enterprise is deploying a large cluster for mission-critical object storage.

• Inputs:
  • Number of OSDs: 60
  • Size per Disk: 16 TB
  • Replication Factor: 3 (standard for production)
• Results:
  • Raw Capacity: 960 TB (60 disks * 16 TB)
  • Usable Capacity (Pre-Overhead): 320 TB (960 TB / 3)
  • Effective Usable Capacity (~85%): ~272 TB

This demonstrates why proper storage cluster calculator usage is essential; the company needs almost 1 Petabyte of raw storage to effectively store 272 TB of data.

How to Use This Ceph Storage Calculator

Using this calculator is simple and intuitive. Follow these steps to estimate your cluster’s capacity:

1. Enter the Number of OSDs: Input the total quantity of physical disks you plan to use in your cluster.
2. Provide the Disk Size: Enter the capacity of a single disk.
3. Select the Unit: Choose whether the disk size you entered is in Terabytes (TB) or Gigabytes (GB). The calculator will handle the conversion automatically.
4. Set the Replication Factor: Specify your desired level of data redundancy. A value of ‘3’ is recommended for production systems to tolerate the loss of two replicas without data loss.
5. Review the Results: The calculator instantly updates the “Effective Usable Capacity”, “Raw Capacity”, “Usable (Pre-Overhead)” capacity, and the associated “Data Redundancy Cost”. The bar chart also adjusts to provide a quick visual comparison.

Key Factors That Affect Ceph Storage

1. Replication vs. Erasure Coding: This calculator focuses on replication, which is performance-oriented. Erasure Coding is an alternative that provides similar durability with less storage overhead but can be more CPU-intensive. Using a Ceph erasure coding calculator is recommended for archival workloads.
2. OSD Count and Size: The total number and size of disks directly determine your raw capacity, which is the starting point for all other calculations.
3. Failure Domain: Ceph’s CRUSH algorithm intelligently places data replicas across different failure domains (e.g., hosts, racks). A proper failure domain setup is critical for achieving the theoretical durability of your replication factor.
4. PG (Placement Group) Count: While not a direct capacity factor, the number of Placement Groups per OSD affects data distribution and resource usage. An incorrect PG count can lead to imbalanced OSDs.
5. Full/Nearfull Ratios: Ceph has safety ratios (typically 85% for `nearfull` and 95% for `full`) that prevent the cluster from running out of space, which is a catastrophic event. Our calculator uses this principle for the “Effective Capacity” estimate.
6. Hardware Homogeneity: While Ceph can handle mixed-capacity disks, clusters are easier to manage and predict when all OSDs are the same size. Uneven disk sizes can lead to unbalanced data distribution.

Frequently Asked Questions (FAQ)

1. What is the difference between raw vs usable Ceph capacity?

Raw capacity is the sum of the storage of all physical disks in the cluster. Usable capacity is the space available after accounting for data protection overhead, like replication or erasure coding. For example, with 3x replication, 3TB of raw storage provides 1TB of usable capacity.

2. What is a good replication factor?

For production environments, a replication factor of 3 is the industry standard. This allows the cluster to tolerate the failure of two disks/nodes holding an object’s replicas without any data loss. For non-critical data, a factor of 2 may be acceptable.

3. Why shouldn’t I fill my cluster to 100% of usable capacity?

Filling a Ceph cluster completely can lead to severe performance degradation and prevent Ceph’s self-healing mechanisms from functioning. If a disk fails, there is no empty space for the data to be re-replicated. Keeping utilization below 85-90% is a critical best practice.

4. Does this calculator work for Erasure Coding?

No, this calculator is specifically designed for replicated pools. Erasure coding has a different overhead calculation based on data (k) and parity (m) chunks (e.g., k=8, m=3). Use our dedicated Ceph raw vs usable space calculator for EC planning.

5. How many OSDs do I need to start a Ceph cluster?

A minimum of 3 OSDs is required for a basic, stable cluster with a standard replication size of 3, with each OSD ideally in a different host to create separate failure domains.

6. What happens if a disk fails?

When a disk fails, Ceph marks its OSD as “down”. It then automatically begins “re-replicating” the data that was on the failed disk to other OSDs in the cluster to restore the target replication factor. This process requires available free space.

7. Can I mix different disk sizes in my cluster?

Yes, Ceph’s CRUSH algorithm can handle heterogeneous hardware. However, it can lead to uneven OSD utilization, where smaller disks fill up faster than larger ones. For predictable performance and simplified management, homogeneous clusters are recommended.

8. Does network speed affect capacity?

Network speed does not affect storage capacity, but it is critical for performance. Slow networks can become a bottleneck during normal operations and especially during recovery or rebalancing events. A minimum of 10Gbps is recommended for the cluster network.